To boost productivity in the Piping Plovers (Charadrius melodus) breeding in the northern Great Plains, predator exclosure “cages” constructed of wire mesh fence were placed over 1,355 plover nests on alkali lake beaches in Alberta, Saskatchewan, North Dakota, and Montana during 1993-2002. Nesting plovers were killed, apparently by raptors, near cages at 68 (5%) of the nests. In contrast, no losses of adult plovers were detected at 420 nests that were not covered by cages. The predation was greatest (up to 48% of applications) when small (1-1.7 m) diameter cages with wire mesh tops were used at sites with low (mean, 4%) or moderate (15%) tree cover within two km. In areas with low tree cover, predation decreased to 0.7% of applications/year when large (3-4 m) diameter cages with soft netting tops replaced other designs. No predation was recorded in 393 applications of small cages at plover nests along the relatively treeless North Dakota-Montana border. Predator exclosure cages should be used cautiously for protecting eggs of endangered shorebirds. In some situations, enhanced productivity from use of the cages is outweighed by risks to adult birds.